1. Field of the Invention
This invention relates to a so-called gradient index lens in the medium of which the refractive index varies.
In particular, the present invention relates to a biconvex lens provided with a pair of convex surfaces sharing the center of curvature, such as a spherical lens.
2. Related Background Art
A spherical lens called a Luneburg lens is well known as a gradient index lens having a sphere-symmetrical index gradient. The index gradient of the Luneburg lens is represented by N(.rho.)=.sqroot.2-.rho..sup.2 (the radius is 1), where .rho. is the distance from the center of the sphere, and the refractive index at the center of the sphere in 1.41 and the reflective index near the surface is 1. The disadvantages peculiar to the gradient index lens are that (1) the refractive index thereof is too low and no suitable material is available, and (2) the refractive index difference between the center of the sphere and the outermost periphery of the sphere is as great as about 0.41 and it is difficult to provide the lens with an index gradient, and these disadvantages have led chiefly to the problems in the manufacture. As a means for solving the former of these disadvantages of the Luneburg lens, it occurs to mind to fill one of the light beam incidence and emergence surfaces of the lens with a medium of a refractive index N.sub.0 with the index gradient of the spherical lens as N(.rho.)=N.sub.0 .sqroot.2-.rho..sup.2, and this is disclosed, for example, in U.S. Pat. No. 4,327,963. Again in this case, however, the refractive index difference between the center of the sphere and the outermost periphery of the sphere is as great as 0.6, and the gradient index lens disclosed therein could hardly be realized.